The present invention relates to a biologically degradable polymer mixture, consisting essentially of starch and at least one hydrophobic polymer, a method of producing a biologically degradable polymer mixture consisting essentially of starch and at least one hydrophobic polymer, as well as a single-layer or multi-layer sheet.
For the control the problem of waste, plastics are more and more as time passes being taken into account, not least of all because their proportion in the treatment of waste is constantly increasing as a result of their increasing use as materials and/or semifinished materials or raw materials. In contradistinction to other materials, such as, for instance, glass or metal, the recycling of plastics, although frequently attempted, is very problematical since it is practically impossible to return the very different types of plastics separately, which in itself would be necessary in order to be able to make reasonable again use of the plastics.
For this reason, it must be assumed that larger and larger amounts of plastic must be disposed of, so that the importance of so-called degradable plastics or polymers is increasing. In this connection, there are of particular interest the biologically degradable polymers which are degraded under the action of microorganisms, bacteria, fungi, enzymes or the like, and as low-molecular products scarcely constitute a problem any longer.
The most widely used biopolymers are in most cases based on starch, in which connection, to be sure, starch is scarcely suitable as an industrially usable polymer. Polymer mixtures with starch are known in which the starch or else other biopolymers are used preferably due to their biological degradability, due to their favorable price, and due to the lack of dependence on petroleum products.
The use of biopolymers as industrial plastic is described in various patents. EP-011 82 40, EP-28 24 51, EP-28 89 20, EP-30 44 01, EP-32 65 14, BG-22 14 919 and EP-32 75 05 describe how native starch, gelatin or other biopolymers are modified or destructured so that they can be used as industrial polymers. This is done by extruding or kneading the starch or the biopolymer with the aid of the water absorbed in the biopolymer or the starch, or by addition of further water, in which case, however, the relatively high water content present is extremely undesired, for instance upon the extrusion of the biopolymer which has been destructured in this manner.
In contradistinction to this, it is proposed in PCT/WO90/05161 that native starch be converted into an industrially usable polymer, i.e. into so-called thermoplastically processable starch without water and with the use of addition substances or softeners or swelling agents, such as, for instance, glycerol. The processability and water resistance of this starch polymer is, it is true, substantially better than the starch polymers produced in accordance with the methods of manufacture described above, but the mechanical properties and, in particular, the water resistance are still insufficient for many industrial uses, such as, in particular, the manufacture of sheets.
For this reason, it has been proposed in a large number of patents that the starch be treated with further polymers in order in this way to obtain properties which permit industrial use. Thus, CH-644 880, CH-644 881, DE-PS-24 62 802 and DE-24 55 732 propose subjecting the starch first of all to a surface treatment with additives and then mixing it with thermoplastic polymers, such as polyethylene, etc. In DE-OS-40 38 732 a material is proposed which has a base of starch with the addition of water and plasticizing agents as well as an aqueous polymer dispersion, formed from a thermoplastic polymer.
In contradistinction to the above-mentioned documents, patents such as EP-40 28, 26, EP-40 47, 23, EP-40 47 27, EP-40 27 28, EP-40 73 50, EP-40 97 88, EP-41 78 28, EP-O 32 802, U.S. Pat. No. 3,952,347, U.S. Pat. No. 5,095,054 and AT-365 619, instead of starting from native starch, start from destructured starch, gelatinized starch or plasticized starch, which are mixed with thermoplastic plastics such as polyolefins, polyalcohols, ABS-copolymers, ethylene/acrylic acid copolymers, copolymers of vinyl pyrrolidines, copolymers with amino groups, etc. In the case of all the above-mentioned polymer mixtures, based on starch, the problem arises that the compatibility between starch and the other polymers is insufficient or the phase mixing of the individual polymers is poor. As is known, starch is extremely hydrophilic, while, as a rule, the polymers used for the mixing are hydrophobic. In the final analysis, the biological degradability of all the said polymer mixtures is still insufficient since the degradation time of the polymers used, which generally are synthetic polymers, is much too long, so that one can scarcely speak of biological degradability.
In order to improve the mechanical properties, it is proposed in a number of patents, such as WO90/10671, WO91/02023 and WO91/02025, that the starch be first of all destructured with a high water content and the water content then reduced to less than 6% upon the production of the polymer mixture in the mixing process with an ethylene copolymer. In WO92/02363 it is furthermore proposed to adding a high-boiling plasticizing agent to the polymer mixture, while in WO92/01743 of the same applicant a boron compound is admixed with the polymer mixture. In DE-39 39 721 it is furthermore proposed to add a cross-linking agent in addition to a polyvinyl/alcohol/starch mixture for the production of sheets.
In order to increase the biological degradability, WO88/06609, in its turn, proposes admixing an iron compound as well as a fatty acid to a polymer mixture consisting of polyethylene and starch or modified starch. EP-282 368, in its turn, proposes a biologically degradable mixture prepared from polyurethane, PVC with plasticizer, a carbohydrate such as starch, as well as an aliphatic polyester, polyurethane being suggested inter alia, based on polycaprolactone. In all the above-mentioned documents of the prior art, it is clear that in all cases some properties are improved by the solution selected while others suffer thereby. Thus, a compromise is always made between the properties, such as water resistance, biological degradability, and mechanical properties in order to be able to improve some of these properties.
Thus, for example, in the production of sheets from a polymer mixture, it is proposed, in accordance with a large number of the above-indicated documents, that the non-starch part of polymer present in the proposed mixtures, produced by itself alone as sheet, i.e. developed with accordingly a substantially thinner wall thickness, has a substantially better tear strength than the thicker sheets produced in corresponding manner from the entire polymer mixture. The reason for this lies either in the high absorption of water or else in the poor phase mixing between the different polymers.
In accordance with patents EP-40 05 31 and EP-40 05 32, it is proposed that the starch be treated, similar to WO90/05161, by a high-boiling plasticizer such as glycerol, urea or sorbitol, in order to obtain thermoplastically processable starch, which is then mixed with ethylene/acrylic acid and/or polyvinyl-alcohol copolymers. Sheets prepared from these starch/polymer mixtures have, it is true, a better tear strength but are still excessively sensitive to moisture.
The same is true of starch/polymer mixtures prepared in accordance with WO90/14388, in which glycerol-starch-polyethylene and ethylene-acrylic-acid copolymers are admixed.
In WO91/16375, in its turn, there is proposed for the production of sheets, a polymer blend which uses thermoplastically processable starch in accordance with WO90/05161, together with a polyoletin, as well as a phase or bonding mediator in order to improve the phase mixing between the starch and the polyolefin. Similarly, in the still unpublished PCT/CH92/00091, it is proposed that thermoplastically processable starch be mixed with at least one polyolefin and at least one ethylene-acrylate-maleic anhydride copolymer, the esterification product between the starch and the added copolymer serving as phase mediator between the starch phase and the polymer phase. The advantage of this polymer mixture is that, due to the good mixing of the starch phase with the further polymer phase, such as for instance the hydrophobic polyolefin phase, excellent mechanical properties can be obtained upon the production, for instance, of sheets.
To be sure, all of these polymer mixtures also have the important disadvantage that their biological degradability can in no way satisfy the demands made today as to such degradability.
In connection with biologically degradable polymers, a direction which is targeted away from starch has therefore been pursued for some time, in the manner, for instance, that aliphatic polyesters the polycaprolactone or polyhydroxybutyric acid/hydroxyvaleric acid copolymers (Biopol) have been developed which can be excellently biologically degraded. The problem with these products, however, resides in the fact that they are very expensive to produce, so that their use as industrial mass-produced plastic does not enter into question.
In this connection, reference may be had to an article in the journal Capital 6/92, p. 208, entitled "Miracle Films Reduce Plastic Rubbish", in which such new developments are described. Another great advantage of these newly developed materials resides in the fact that they are hydrophobic and thus water resistant, this in contradistinction to starch which is very hydrophilic.
This difference brings it about, however, that a "stretching" of the said expensive, biologically degradable polymers with starch involves the problem that two immiscible phases are present. This non-miscibility of starch with the said hydrophobic, biologically degradable polymers, in its turn, has the result that mixed polymers produced in this manner have poor mechanical properties, whereby, once again, the problem discussed originally above is present, namely that compromises must be made between water-solubility, biological degradability, and mechanical properties.